1) Field of the Disclosure
The disclosure relates to film adhesives. More particularly, the disclosure relates to a multi-layer film adhesive with at least one layer having a chemical or adhesive property that is different than a chemical or adhesive property of another layer.
2) Description of Related Art
Multi-ply film adhesives are often used to join or bond two materials together, such as composite and/or metal components of aircraft, rotorcraft, vehicles such as automobiles and trucks, items of equipment, architectural structures, and various other structures having component parts to be adhesively bonded. Such adhesives may be used to bond two composite materials together, two metal materials together, a composite material to a metal material, a pre-cured material to an un-cured material, or a pre-cured material to a pre-cured material. Known adhesives, such as nitrile phenolics and epoxies, may be used to bond metal materials to metal materials, such as metal honeycomb structures. However, such nitrile phenolics have proven not to be optimal at bonding two composite materials together, a composite material to a metal material, a pre-cured material to an un-cured material, or a pre-cured material to a pre-cured material.
Known film adhesives typically consist of a single homogeneous layer that is optimized for a single adhesive or chemical property, such as gap filling, wetability, flow rate, bond strength, material compatibility, temperature versus viscosity, ease of application, or another adhesive or chemical property. For example, a difficulty with bonding composite or metal parts together is that gaps may form and vary from place to place on a single structure such as an aircraft, and thus the use of adhesives with good gap filling properties can prevent or minimize such gaps. However, the use of such known single layer adhesives with good gap filling properties may require an excessive amount of such adhesive to be applied to the parts, thus resulting in wasted adhesive that must be cleaned up and removed. This can be labor intensive and time consuming, resulting in increased manufacturing costs. In addition, a difficulty with bonding composite or metal parts to a substrate is that although a known single layer adhesive may wet or flow well over the surface of the substrate so that the composite or metal effectively adheres to the substrate, such adhesive may not have sufficient strength to maintain joint integrity. Similarly, a known single layer adhesive may exhibit good specific strength properties but may not have good wetability or flow to adequately wet the surface of the substrate to maintain joint integrity. Moreover, known single layer adhesives can produce excess adhesive squeeze out when composite or metal parts are bonded together using such adhesives. Where such excess adhesive squeeze out can be seen and accessed, removal of such excess adhesive squeeze out requires additional labor and time to remove. This can increase manufacturing costs. Where such excess adhesive squeeze out cannot be seen or accessed, such as in a closed assembly, removal of such excess adhesive squeeze out may not be possible and thus, such excess adhesive squeeze out remains on the part and adds extra weight to the part, such as an aircraft component part. Such extra weight on an aircraft component part may increase the weight of an aircraft and decrease the aircraft's performance.
Accordingly, there is a need for a multi-layer film adhesive that has advantages over known film adhesives.